Damage due to electrostatic discharge (ESD) and/or electrical overstress (EOS) costs industry uncounted and perhaps uncountable dollars daily in damaged and irreparable goods. More specifically, ESD/EOS damage is a particular problem in the electronics industry where the components are, of course, designed to conduct electricity in the first instance and where their continuously shrinking size renders them increasingly susceptible to such damaging effects. Generally, ESD refers to actual discharges while EOS refers to the effects of such discharges or currents induced by such discharges or other electrical or magnetic fields. For present purposes, reference to one should be interpreted to include the other.
ESD, familiarly manifested by the lightning bolts or by the shock received when touching a door knob, after walking across a carpet, can range from a few volts to as much as several thousand volts, resulting in extremely large transient currents. As electronic components shrink in size they become ever more susceptible to damage from smaller and small voltages and current.
ESD can arise in several different ways, most commonly as a result of triboelectric charging or induction. Triboelectric charging causes a charge build up due to the frictional engagement of two objects. That is, static charge builds up as a result of a series of contacts and separations of two objects. Electrons travel from one object to the other during these contacts depending on the relative abilities of the objects to gain or lose electrons, that is, depending upon the position of the two objects in the electrochemical potential series. Consequently, a net charge of opposite sign will build up and remain on both of the objects after their separation. Where the object has good conductivity and is grounded, charge will flow to the ground. If the electric field generated by the separated charges is strong enough, an electrostatic discharge can occur in form of a spark traveling across an air gap from one object towards an object at a lower electrostatic potential, thus providing the familiar blue light generated by the spark. This discharge can occur either as one object is brought next to one of the charged objects or as one object is separated from the other.
Static charges can also build up by induction. That is, if a charged object is brought near an uncharged object, the electric field of the charged object will induce a charge in the object, generating an electric field and potentially a static discharge.
A goal in many industries, then, is to determine methods and apparatus for reducing or eliminating static discharges. One of the electronics industries affected by ESD/EOS damage is that which manufactures and assembles computer hard disk drives. As noted above, present hard disk drives include a disk rotated at high speeds and a read/write head that, in industry parlance, "flies" a microscopic distance above the disk surface. The disk includes a magnetic coating that is selectively magnetizable. As the head flies over the disk, it "writes" information, that is, data, to the hard disk drive by selectively magnetizing small areas of the disk; in turn, the head "reads" the data written to the disk by sensing the previously written selective magnetizations. The read/write head is affixed to the drive by a suspension assembly and electrically connected to the drive electronics by an electrical interconnect. This structure (suspension, electrical interconnect, and read/write head) is commonly referred to in the industry as a Head Gimbal Assembly, or HGA.
More specifically, currently manufactured and sold read/write heads include an inductive write head and a magnetoresistive (MR) read head or element or a "giant" magnetoresistive (GMR) element to read data that is stored on the magnetic media of the disk. The write head writes data to the disk by converting an electric signal into a magnetic field and then applying the magnetic field to the disk to magnetize it. The MR read head reads the data on the disk as it flies above it by sensing the changes in the magnetization of the disk as changes in the voltage or current of a current passing through the MR head. This fluctuating voltage in turn is converted into data. The read/write head, along with a slider, is disposed at the distal end of an electrical interconnect/suspension assembly.
Other types or read heads, such as inductive read heads, are known, but the MR and GMR elements enable the reading of data that is stored more densely than that which was allowed with the use of inductive read element technology. MR and GMR read elements are much more sensitive to current transients resulting from voltage potentials and thermal gradients, however, than the previous read element technologies. It is now becoming increasingly necessary to manage environmental electrostatic charge levels to as low 3.3 volts during HGA manufacturing processes so as not to damage the MR and GMR elements. Failing to do so, or failing to provide an avenue for the safe discharge of the accumulated electrostatic charge can result in damage to the MR and GMR heads.
Damage to an MR or GMR head can be manifested as physical damage or magnetic damage. In the former, melting of the read element in the head can occur because of the heat generated by the transient current of the discharge. Magnetic damage can occur in the form of loss of sensing ability and instability. Furthermore, direct discharge into the head is not necessary to create the damage. Damaging current flows in the head can also reportedly be created through electromagnetic interference as a result of a distant (relatively speaking) discharge.
An exploded view of a typical electrical interconnect/suspension assembly is shown in FIG. 1, which illustrates several components including a suspension A and an interconnect B. It will be understood that the actual physical structures of these components may vary in configuration depending upon the particular disk drive manufacturer and that the assembly shown in FIG. 1 is meant to be illustrative of the prior art only. Typically, the suspension A will include a base plate C, a radius (spring region) D, a load beam E, and a gimbal F. At least one tooling aperture G may be included. An interconnect B may include a base H, which may be a synthetic material such as a polyimide, that supports typically a plurality of electrical traces or leads I of the interconnect. The electrical interconnect B may also include a polymeric cover layer that encapsulates selected areas of the electrical traces or leads I.
Stated otherwise, suspension A is essentially a stainless steel support structure that is secured to an armature in the disk drive. The read/write head is attached to the tip of the suspension A with adhesive or some other means. The aforementioned electrical interconnect is terminated to bond pads on the read/write head and forms an electrical path between the drive electronics and the read and write elements in the read/write head. The electrical interconnect is typically comprised of individual electrical conductors supported by an insulating layer of polyimide and typically covered by a cover layer. Prior to the time that the HGA is installed into a disk drive, the electrical interconnect is electrically connected to the read and write elements, but is not connected to the drive electronics. As a result, the individual conductors that make up the electrical interconnect, can easily be charged by stray voltages, thereby creating a voltage potential across the sensitive MR or GMR read elements, which when discharged results in damaging current transients through the read element.
The components shown in FIG. 1 as well as all those associated with hard disk drives are small and continually decreasing in size. Consequently, any tolerance for ESD/EOS damage of the components during the assembly process is also continuously decreasing while their susceptibility to damage during assembly is increasing.
As noted, an ESD can actually damage or destroy circuit pathways in small electronic parts, such as an MR head, requiring the head to be discarded. The industry has been so concerned about this costly manufacturing problem that numerous patents have issued addressing the problem, including but not limited to U.S. Pat. Nos. 5,867,888 for Magnetic Head/Silicon Chip Integration Method; 5,855,301 for Electrostatic Grounding System for a Manually Operated Fluid Dispenser; 5,843,537 for Insulator Cure Process for Giant Magnetoresistive Heads; 5,837,064 for Electrostatic Discharge Protection of Static Sensitive Devices Cleaned with Carbon Dioxide Spray; 5,812,357 for Electrostatic Discharge Protection Device; 5,812,349 for Magnetic Head Apparatus Including Separation Features; 5,761,009 for Having Parastic [sic] Shield for Electrostatic Discharge Protection; 5,759,428 for Method of Laser Cutting a Metal Line on an Mr Head; 5,757,591 for Magnetoresistive Read/Inductive Write Magnetic Head Assembly; Fabricated with Silicon on Hard Insulator for Improved Durability and Electrostatic Discharge Protection and Method for Manufacturing Same; 5,757,590 for Fusible-Link Removable Shorting of Magnetoresistive Heads for Electrostatic Discharge Protection; 5,748,412 for Method and Apparatus for Protecting Magnetoresistive Sensor Element from Electrostatic Discharge; 5,742,452 for Low Mass Magnetic Recording Head and Suspension; 5,732,464 for Method of Facilitating Installation or Use of an Electromechanical Information-Storage Device Drive Assembly; 5,710,682 for Electrostatic Discharge Protection System for Mr Heads; 5,699,212 for Method of Electrostatic Discharge Protection of Magnetic Heads in a Magnetic Storage System; 5,686,697 for Electrical Circuit Suspension System; 5,654,850 for Carbon Overcoat with Electrically Conductive Adhesive Layer for Magnetic Head Sliders; 5,650,896 for Low Cost Plastic Overmolded Rotary Voice Coil Actuator; 5,644,454 for Electrostatic Discharge Protection System for Mr Heads; 5,638,237 for Fusible-Link Removable Shorting of Magnetoresistive Heads for Electrostatic Discharge Protection; 5,589,777 for Circuit and Method for Testing a Disk Drive Head Assembly Without Probing; 5,491,605 for Shorted Magnetoresistive Head Elements for Electrical Overstress and Electrostatic Discharge Protection; and 5,465,186 for Shorted Magnetoresistive Head Leads for Electrical Overstress and Electrostatic Discharge Protection During Manufacture of a Magnetic Storage System.
The foregoing patents generally evidence four different methods for reducing or eliminating ESD damage to MR heads, each relying upon the minimization of the voltage potential across the read elements or dissipation of the static electric charge--that is, the creation of an electrical short--and not the prevention of its buildup in the first instance. These methods include the use of mechanical clips, solder bridges, conductive tape, or a tear-away or sheared etched electrical shunt that is manufactured into the HGA by vapor deposition and etching or some other process. While each of these methods has met with some success, each has its own particular disadvantages. For example, mechanical clips are relatively expensive and also require a substantial amount of manual labor to attach them to the electrical interconnect; solder bridges arc difficult to attach and then remove without causing damage to sensitive parts, can be a source of contamination in the drive, and also require manual labor for solder application and removal; conductive tape is expensive and requires manual labor for application; and tear away shunts require expensive apparatus, prohibit the electrical interconnect manufacturer from performing badly needed in-process continuity checks on the electrical interconnect, and is intentionally designed as a one-time shunt.
There are disadvantages that are shared by all of the above methods. First, each method is essentially a one-time application of an electrical short. That is, each of these methods relies upon a one-time placement and subsequent removal of the electrical short. Preferred manufacturing and quality testing operations, however, may require the successive application and removal of electrical shorts. For example, prior to in-process read/write head characterization, the electrical interconnect must be de-shunted, and then re-shunted after the head characterization to prevent ESD damage later in the manufacturing process. Yet, as noted, most of the foregoing methods of providing shunts are limited in their ability to be reapplied. This inability to repeatedly create and remove electrical shorts as desired is a critical limitation in present manufacturing operations. In addition, the very act of placing and, particularly, removing the electrical short can cause the very ESD sought to be avoided and, therefore, the damage that the short was to prevent in the first instance.
Further, each of the foregoing methods relies upon a physical engagement with the critical components of the MR head with at least one and sometimes two or more physical contacts, at least with the shunt itself and also, depending upon the shunting method used, with the tool applying the electrical shunt itself to the head. Each of these engagements and disengagements carries with it the potential for damaging the head.
Broadly stated, it would be desirable to have a method of creating and removing electrical shorts as desired in sensitive electronic components that did not depend upon a physical application of a conductive circuit to the component. More specifically, it would be desirable to have a method of creating and removing an electrical short to prevent ESD/EOS damage in an MR head when desired and any number of times desired.